Induced polarization mapping with borehole to surface electromagnetic transmitter data

ABSTRACT

An electromagnetic energy transmitter is provided for induced polarization mapping with a borehole to surface electromagnetic (BSEM) transmitter. The transmitter is lowered into a well borehole to a desired location in a well borehole to emit electromagnetic energy. An array of receivers spaced over an area of the earth surface senses the induced polarization response to the emitted electromagnetic energy in areas of interest for mapping. Maps are formed of the induced polarization responses sensed by the array of receivers.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of Applicant's co-pending U.S.patent application Ser. No. 13/090,691, filed Apr. 20, 2011, priorityfor which is claimed pursuant to 35 U.S.C. 120, which has been issued asU.S. Pat. No. 8,689,866, dated Mar. 25, 2014.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electromagnetic energy source ortransmitter for borehole to surface electromagnetic surveying andmapping of subsurface formations.

2. Description of the Related Art

Electromagnetic methods to obtain data regarding subsurface earthformations and their constituent fluid contents have been used forseveral purposes. Among these have been petroleum reservoircharacterization and front-tracking in enhanced oil recovery operations.

One of these electromagnetic methods has been what is known as theborehole-surface or borehole to surface electromagnetic method (BSEM).Two electrodes have been used in the borehole-surface electromagneticenergy method. The first electrode has been in a well borehole of whatis known as the transmitter well, transmitting electromagnetic energy,and the other, which may be a ground electrode, has been at the earth'ssurface along with a receiver array. The receiver array has been locatedat spaced positions on the surface conforming to the reservoir ofinterest to detect the energy field after passage through the earth fromthe first or transmitter electrode.

In a typical operation, Borehole to Surface Electromagnetic (BSEM)utilized an electromagnetic source in the borehole and an array(typically 600-2000 or more) of receivers on the surface, thus allowingthe mapping of the fluid (typically oil and water) distribution in largeareas of the reservoir a few (2-4) kilometers away from the well inwhich the transmitter electrode had been positioned.

The transmitter electrode located in the well was activated at depths ofinterest. The signal emitted on activation could be a single frequencyor multiple frequencies. The resultant electromagnetic field which thenoccurred was sensed in the time and frequency domains by the receiverarray. Surveys of this type could then be repeated after passage of aperiod of time from the transmitter well to track the subsurface fluidmigration.

An interface in a subsurface formation between solids and liquidsproduces induced polarization and frequency scattering responses to theemitted signals and the responses were received and recorded. Therecorded data was processed and analyzed to map boundaries of subsurfacereservoirs of interest and evaluate other nearby formations. Theinformation obtained was important in assessing the sweep efficiency, orthe percentage of original oil displaced from a formation by a floodingfluid, and in locating potential bypassed oil zones, thus ultimatelyincreasing oil recovery.

So far as is known, no provision has been made to obtain a preciselyaccurate measurement of the depth position of the transmitter downhole.An indirect measurement was possible only from measurements of thelength of cable passing from the cable reel or drum in the wirelinetruck into the well. However, this length measurement did not take intoaccount elongation of the cable at increasing depths in the well. Thisgave rise to an inability to accurately determine well depthmeasurements of formations and correlate actual depth of the transmitteremissions of energy with data representative of subsurface conditions.

During BSEM surveying, other well logging operations with other welllogging tools present in the well borehole were not, so far as is known,conducted. The purpose of this was so that the transmitter electrodecould be easily moved to desired depths in the well. Thus, there was nocapability to monitor downhole well conditions during the BSEM survey.Thus, so far as is known, no provision was made to detect incipientabnormal conditions which might provide advance notice of one or more ofpossible problems, such as overheating of the transmitter electrode,starting of an ignition in the well, a gas kick, overpressure, or thelike.

SUMMARY OF THE INVENTION

Briefly, the present invention provides a new and improvedelectromagnetic energy transmitter mounted with a wireline forelectromagnetic surveys of subsurface earth formations from a wellborehole which has a casing installed along its extent into the earth toa location near a formation of interest, the casing being formed oflengths of tubular members connected at end portions to adjacent tubularmembers by casing collars. The electromagnetic energy transmitterincludes an electromagnetic energy source emitting electromagneticenergy in the form of electric current when activated, and a controlcircuit activating the conductive bar to emit electromagnetic energy fora selected time and duration. The electromagnetic energy transmitteralso includes a sonde body housing the control circuit. The sonde bodyis adapted to be lowered by the wireline in the well borehole to thelocation near the formation of interest. An upper connector subassemblyis mounted above the sonde body connecting the control circuit to thewireline and permits the flow of electrical current to theelectromagnetic energy source. A lower connector subassembly is mountedbelow the sonde body and connects the electromagnetic energy source tothe control circuit. The electromagnetic energy transmitter alsoincludes a casing collar locator mounted in the sonde body to provideindications of movement of the sonde body past casing collar in thecasing during movement of the transmitter through the well borehole. Theelectromagnetic energy transmitter further includes a fluid pressuresensor mounted in the sonde body for measuring fluid pressure in thewell borehole at the location of the sonde body; and a temperaturesensor mounted in the sonde body for measuring temperature in the wellborehole at the location of the sonde body.

The present invention also provides a new and improved method ofelectromagnetic surveying subsurface earth formations from a wellborehole which has a casing installed along its extent into the earth toa location of interest near a formation of interest, the casing beingformed of lengths of tubular members connected at end portions toadjacent tubular members by casing collars. According to the presentinvention electromagnetic energy source with a sonde body connectedtherewith is lowered to the location of interest in the borehole. Ameasure is formed with the casing collar locator of the number of casingcollars past which the source and sonde body travel during the step oflowering to determine the depth of the source and sonde body in theborehole based on the measured number of casing collars. The casingcollar locator is then deactivated when the source and the sonde bodyare at the location of interest. Electromagnetic energy is then emittedfrom the source at the location of interest to travel through thesubsurface formations for electromagnetic energy surveying of thesubsurface earth formations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram, taken partly in cross-section, of aborehole to surface electromagnetic survey system disposed in a wellborehole to obtain borehole to surface electromagnetic survey dataaccording to the present invention.

FIG. 2 is an enlarged view of a portion of the well casing of thestructure illustrated in FIG. 1.

FIG. 3A is a schematic diagram of an upper portion of a borehole tosurface electromagnetic transmitter according to the present invention.

FIG. 3B is a schematic diagram of an intermediate portion of a boreholeto surface electromagnetic transmitter according to the presentinvention.

FIG. 3C is a schematic diagram of a lower portion of a borehole tosurface electromagnetic transmitter according to the present invention.

FIG. 4 is an example display of well log data from conventional welllogs regarding subsurface formations as a function of depth in a wellborehole.

FIG. 5 is a plot of induced polarization data obtained from subsurfaceformations over a range of depths during borehole to surfaceelectromagnetic surveying and mapping of subsurface formations adjacentthe well borehole in which the well log data of FIG. 4 was obtained.

FIG. 6 is a plot of induced polarization data obtained from subsurfaceformations over a different range of depths than FIG. 5 during boreholeto surface electromagnetic surveying and mapping of subsurfaceformations adjacent the well borehole in which the well log data of FIG.4 was obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, a borehole to surface electromagnetic (BSEM) surveysystem B is shown schematically in FIG. 1 in connection with a wellborehole 10 which has been drilled into the earth through rock insubsurface earth formations F having hydrocarbon fluids of interest. Anelectromagnetic energy transmitter T (FIGS. 3A, 3B and 3C) according tothe present invention is mounted with a wireline 12 for electromagneticsurveys of the subsurface earth formations F from the well borehole 10.As is typical, the well borehole 10 has a casing 14 (FIGS. 1 and 2)installed along its extent into the earth to a location near areservoir. A typical casing string 14 extends several thousands of feetfrom wellhead 15 at or above ground level to a lowermost casing sectionor casing shoe 16 within the wellbore 10. Below the depth of the casingshoe at 16, the lower portion of the well where no casing is present iswhat is known as open hole 17.

The casing 14 is formed of lengths of tubular joint members 18 (FIG. 2)connected at upper and lower end portions 18 a and 18 b to adjacenttubular members 18 by casing collars 20. The ends of each tubular jointor segment 18 of casing string 14 are externally threaded, and thecollars 20 are internally threaded to mate with the threaded portion ofthe adjacent casing members 18. As is conventional, where two pieces ofcasing pipe 18 are joined with a collar 20, there may in some wells be asmall gap between the adjacent ends of the two sections of casing.Alternatively, in what is known as “flush joint” casing, no gap ispresent between the ends of adjacent casing member sections which areheld in abutting relationship by collar 20.

In connection with borehole to surface electromagnetic (BSEM) surveys,the transmitter T and wireline cable 12 are suitably supported at thewellhead 15 such as by a sheave wheel 22, which is used to raise andlower the transmitter T in the wellbore 10. During the borehole-surfaceor borehole to surface electromagnetic surveys, two electrodes are used.A first electrode 30 (FIG. 3B) of the transmitter T according to thepresent invention is in the well borehole 10, which serves as thetransmitter well to transmit electromagnetic energy of desired frequencyand amplitude into earth formations around the well borehole for travelthrough the subsurface earth formations F. The other electrode 32 (FIG.1), which may be a ground electrode, is at the earth's surface 34 alongwith a receiver array A indicated schematically in FIG. 1. The receiverarray A is composed of electromagnetic energy receivers 36 located atspaced positions on the earth over a surface area conforming todimensions of a reservoir of interest. Receivers in the receiver array Adetect the transmitted energy field after passage through the earth fromthe transmitter electrode T. A borehole to surface electromagneticsurvey allows mapping of the fluid (typically oil and water)distribution in large areas of the reservoir a few (typically 2-4)kilometers away from the well in which the transmitter electrode hadbeen positioned. Parameters of interest in such a survey are resistivityand induced polarization or IP, as will be set forth.

The transmitter electrode T located in the well 10 is activated atdepths of interest. The resultant electromagnetic field is sensed in thetime and frequency domains by the receiver array A. Surveys of this typeare repeatable at required intervals over a period of time to trackmigration of subsurface fluids.

The electromagnetic energy transmitter T includes a conductive metal baror rod 40 of copper or other similar conductive material. The conductiveelectrode energy source 40 is operatively connected to a control circuit42 which responds to control signals sent from the surface from atransmitter vehicle V at the surface over the wireline 12 and activatesthe conductive electrode 40 to emit electromagnetic energy of thedesired frequency and amplitude for a selected time and duration duringBSEM surveying.

According to the present invention, the borehole depths at which theBSEM survey electromagnetic energy is emitted by the transmitter Tduring surveys are obtained in a manner to be set forth. The boreholedepth readings are recorded along with the sensed electromagnetic fieldscorresponding to emissions at that depth in a suitable data memory in acomputer or data processor in a logging vehicle or truck L (FIG. 1).Once recorded, the BSEM data and depth measurements are transferred asneeded into the data processing system or computer for on siteprocessing and analysis and are available for further processing andanalysis elsewhere. Records of the time and content of theelectromagnetic energy specified by control signals are also furnishedfrom the transmitter vehicle V to data recoding computer or processorequipment in the logging vehicle or truck L.

The electromagnetic energy transmitter T also includes a sonde body 44(FIGS. 3A and 3B) connected to the wireline 12 by an upper connectorsubassembly 46. The transmitter T is adapted to be lowered by thewireline 12 in the well borehole 10 to the various depths indicated asadjacent or near the formations of interest for BSEM surveying. Theupper connector subassembly 46 is mounted above the sonde body 44operatively coupling the control circuit 42 to the wireline 12 toprovide electrical energy as well as mechanical connection for thetransmitter T. The upper connector subassembly 46 permits the flow ofelectrical current to provide power for signals emitted by theelectromagnetic energy source 40 during surveys and passage of controlsignals to the control circuit 42.

With the present invention, the electromagnetic energy transmitter T isprovided with a casing collar locator 50 mounted within the sonde body44 and electrically connected through connector subassembly 46 andwireline 12 with surface electronics in the logging vehicle L to provideindications of movement of the transmitter T and sonde body 44 pastcasing collars 20 in the casing string 14 during movement of thetransmitter T through the well borehole 10. The casing collar locator 50may be one of several available types, such as those available fromSondex (General Electric Co.) of Hampshire, UK. In the casing collarlocator 50, magnetic sensors detect the presence of casing collars 20 bysensing larger metallic mass at the location of the casing collar at theends of the sections 18 of casing than along the length of the casingsections 18.

Electronic circuitry within the casing collar locator 50 formselectrical signals usually in the form of pulses as the locator passessuccessive casing collars 20 during movement of the transmitter Tthrough the well borehole 15. The casing collars 20 are located atdefined known lengths from each other according to the known distance orlength of a casing section 18 between its ends 18. Thus a count of thenumber of casing collars 20 passed during movement of the transmitter toa target depth such as shown at 52 or 54, for example, either in theopen hole region 17 or within the casing string 14 indicates accuratelyfor the purposes of the present invention the depth of the transmitterT. The casing collar locator 50 thus measures the position of thetransmitter T relative to the last casing point or casing shoe at depth16. The casing collar locator 50 is provided with on-off switchingcapability so that measurements are not being made with the locatorduring the transmission of electromagnetic signals from the transmitterT. Thus, the casing collar locator 50 is sensing and transmittingsignals indicating the presence of casing collars only at those timeswhen the locator is passing through the casing shoe 16 before enteringin the target zone.

The transmitter T of the present invention thus compensates for anypotential bias or distortion in the accuracy of depth locations at whichthe transmitter T is activated which are induced by the elongation ofthe wireline cable from surface to the target depth. This has been foundto be satisfactorily accurate even when the transmitter is located at adepth in open hole 17. Normally there are only a few feet of open holesection at the end of a cased well. The possible elongation of the cablein the last few feet of open-hole has been found to be negligiblecompared to the thousands of feet in the cased section 14.

The electromagnetic energy transmitter T in accordance with the presentinvention is also provided with a pressure and temperature sensingcapability which includes a fluid pressure sensor 55 and a temperaturesensor 60 mounted in the sonde body 44. The fluid pressure sensor 55measures fluid pressure in the well borehole at the location of thesonde body 44 within the wellbore 10. The fluid pressure sensor 55 iselectrically connected with surface electronics in the logging vehicle Lto provide indications of fluid pressure at the location of transmitterT. The pressure sensor 55 may be one of several available types, such asthose available from Omega Data Services Limited of Aberdeen, Scotland.

The temperature sensor 60 measures fluid pressure in the well boreholeat the location of the sonde body 44 within the wellbore 10. Thetemperature sensor 60 is electrically connected with surface electronicsin the logging vehicle L to provide indications of temperatureconditions at the location of transmitter T. The temperature sensor 60may be one of several available types, such as those available fromOmega Data Services Limited of Aberdeen, Scotland.

According to the present invention, it is now possible to monitor thedownhole conditions of pressure as well as temperature during BSEMsurveys. In this manner, well crews are able to identify and take stepsto prevent a potential problem from occurring. Examples of suchpotential problems are overheating of the transmitter electrode T; anignition starting in the well borehole, a gas kick in the well, anoverpressure condition, and the like. Accordingly, survey crews and wellcrews are able to sense and detect conditions which might give rise tothe risk of blowout or ignition, or might affect the quality of data.

It has also been found that due to the very low electromagneticfrequency typically used in BSEM surveys, energy emitted during thesurveys does not affect the pressure and temperature measurements sensedby the sensors 55 and 60, respectively. The electromagnetic currentcould, however, affect pressure and temperature conditions downhole. Thepresent invention by including pressure sensors and temperature sensorsintegrated in the BSEM transmitter T is able to detect possibleanomalous increase of temperature or pressure, or both, due to a numberof reasons. Examples are overheating of the BSEM transmitter electrodeor antenna 30, with the risk of melting the transmitter T or wirelinecable 12; an anomalous hydrocarbon overpressure bubble entering thewell; and possible ignition of gases started downhole, whether or nottriggered by the electromagnetic current emitted. The pressure andtemperature readings sensed with the present invention are important fortimely preventive measures to be taken at the surface, such as stoppingtransmission of the BSEM signals, activating the well controlprocedures, emergency measures as required.

A lower connector subassembly 70 (FIG. 3B) is mounted below the sondebody 44 and connects the conductive metal bar 40 of electrode 30 sourceto the control circuit 42 so that electrical power is provided to themetal bar 40 to emit electromagnetic energy of the desired frequency andamplitude for a selected time and duration during BSEM surveying.

The conductor bar 40 is a solid bar of requisite thickness formechanical strength formed of copper and is, for example about 0.8 m inlength. A weight bar connector 72 is mounted at a lower end of conductorbar 40 to connect a swivel connector subassembly 74 with upper portionsof the transmitter T. The swivel connector subassembly 74 provides asindicated schematically at 76 for pivotal movement and connection of aweight bar member 78 of a suitably heavy material to the upper portionsof the transmitter T. The weight bar member 78 assists, as isconventional, in proper orientation and movement of the transmitter T inthe well borehole 10. Typically, as indicated at 80 a nose plug ismounted below the weight bar 78 for facilitating movement of thetransmitter T through the well borehole 10.

In the operation of the present invention borehole to surfaceelectromagnetic surveying of subsurface earth formations is performed inthe well borehole 10 when the transmitter T with sonde body 44 arelowered to locations of interest in the borehole in the free hole zone17 borehole below the casing 18. A measure is formed, during suchmovement, with the casing collar locator 50 of the number of casingcollars 20 past which the transmitter and sonde body travel duringlowering and the measurements forwarded to the surface over the wireline12 and recorded in the logging truck L. In this manner, the depth of thetransmitter T in the borehole 10 is measured and recorded based on themeasured number of casing collars. The casing collar locator 50 is thendeactivated when transmitter T is at a location of interest.Electromagnetic energy is then emitted from the conductive bar 40 at thelocation of interest to travel through the subsurface formations forelectromagnetic energy surveying of the subsurface earth formations.

FIG. 4 is a simplified example display of well log data fromconventional well logs as a function of borehole depth regardingsubsurface formations as a function of depth in the well borehole 10.The well log or plot in FIG. 4 illustrate as a function of depth over arange of porosity values from below 5% to about 25% the relativepresence of oil as indicated at 100 and water as indicated at 102. Themeasurements from which the data displayed in FIG. 4 were attained froman example well in an existing reservoir.

Another measurement of interest in addition to the well logs of FIG. 4obtainable from the same subsurface formations is data obtainable fromBSEM surveys. One of the parameters obtainable from data from BSEMsurveys of subsurface earth formations from a well borehole is InducedPolarization or IP. Plots or maps of induced polarization for aninvestigative layer in the subsurface formations are utilized indiscriminating oil from water zones in the formations near or evenwithin a few kilometers from the well borehole. If the inducedpolarization maps obtained from BSEM survey data indicate a high inducedpolarization measure, this indicates that there is a high oil saturationin the investigated layer. Conversely, if the induced polarization mapsobtained from BSEM survey data indicate a low induced polarizationmeasure, this indicates that there is a water saturation in theinvestigated layer.

FIG. 5 is a plot or map of induced polarization as a function of surfacearea or extent based on BSEM survey data for a layer indicated asextending from depth A1 through depth A4 in the well which is thesubject of the well logs plotted in FIG. 4. For ease of reference andanalysis, the well log plot is also included in FIG. 5. The inducedpolarization measurements as determined from BSEM surveys are plotted inthe color key 104 for the map of FIG. 5. The locations or depths soindicated in the well of depths A1 and A4 are depicted in the well logplots of FIGS. 4 and 5.

FIG. 6 is a plot or map of induced polarization based on BSEM surveydata for a layer determined according to the present invention asextending from the depth A1 through depth A2 in the same well which isthe subject of the well logs plotted in FIG. 4. For ease of referenceand analysis, the well log plot is also included in FIG. 6. The depth A2is also indicated in the well log plotted in FIG. 4 along with depths A1and A4. The map co-ordinates are plotted in the margins of FIG. 6. As isevident, the areas which are the subject of FIGS. 5 and 6 substantiallyoverlap. The induced polarization measurements as determined from BSEMsurveys are plotted in the color key 104 for the map of FIG. 5 which isthe same as that of FIG. 6.

In the induced polarization maps of FIGS. 5 and 6 the differences ofinduced polarization response are apparent. In the map of FIG. 5, theinduced polarization data map provides indications of oil as indicatedby the areas in the upper right quadrant of the map when the transmitteris located at the layer between depths A1 and A4. Conversely, theinduced polarization data map of FIG. 6 which refers to the layer A1 andA2 indicates the presence of substantially more water in the samegeneral area of the reservoir of interest, with water indicated in thesame reservoir area.

Accordingly, with the present invention a more precise measure andknowledge of the depth where electromagnetic signal energy is beingtransmitted is provided. A more accurate reading of the depth locationof the transmitting antenna or electrode 30 is available. It can be seenthat if the location of the electromagnetic transmitter antenna 30 isproperly indicated at A2 instead of A4 a markedly different map of fluiddistribution is obtained and measured for the selected reservoir layer.It can also be seen that thus it is now possible to have an accuratemeasurement of the depth position of the BSEM transmitter T downhole.

The invention has been sufficiently described so that a person withaverage knowledge in the matter may reproduce and obtain the resultsmentioned in the invention herein. Nonetheless, any skilled person inthe field of technique, subject of the invention herein, may carry outmodifications not described in the request herein, to apply thesemodifications to a determined structure, or in the manufacturing processof the same, requires the claimed matter in the following claims; suchstructures shall be covered within the scope of the invention.

It should be noted and understood that there can be improvements andmodifications made of the present invention described in detail abovewithout departing from the spirit or scope of the invention as set forthin the accompanying claims.

What is claimed is:
 1. A method of mapping the induced polarizationresponse of an investigative layer of a subsurface earth formation at alocation of interest as a function of depth in a well borehole which hasa casing installed along its extent into the earth to a location ofinterest near a formation of interest in the investigative layer, thecasing being formed of lengths of tubular members connected at endportions to adjacent tubular members by casing collars, the methodcomprising the steps of: lowering a borehole to surface electromagneticsurvey transmitter with a sonde body connected therewith to the locationof interest in the borehole below the casing; locating a groundelectrode at the surface of the earth; locating an array of surfaceelectromagnetic field receivers at spaced positions over a surface areaon the earth surface; forming a measure with the casing collar locatorof the number of casing collars past which the source and sonde bodytravel during the step of lowering to determine the depth of the sourceand sonde body in the borehole based on the measured number of casingcollars; deactivating the casing collar locator when the source and thesonde body are at the location of interest; and emitting anelectromagnetic field from the borehole to surface electromagneticsurvey transmitter at the location of interest to travel through thesubsurface formations to the surface and form an electromagnetic field;detecting with the array of surface electromagnetic field receivers atspaced positions over the earth surface a measure of inducedpolarization as a function of borehole depth resulting from the emittedelectromagnetic field from the borehole to surface electromagneticsurvey transmitter at the location of interest; and forming an areal mapof the detected measure of induced polarization for induced polarizationmapping of the investigative layer of the subsurface earth formations atthe location of interest.
 2. The method of claim 1, further includingthe step of: sensing fluid pressure in the well borehole at the locationof the sonde body during the emitting of electric current.
 3. The methodof claim 1, further including the step of: sensing temperature in thewell borehole at the location of the sonde body during the emitting ofelectric current.
 4. The method of claim 1, further including the stepof: moving the electric current source and the sonde body to a changedlocation of interest in the borehole.
 5. The method of claim 4, furtherincluding the step of: emitting an electromagnetic field from theborehole to surface electromagnetic survey transmitter at the changedlocation of interest.
 6. The method of claim 5, wherein the location ofinterest is in the casing in the well borehole.
 7. The method of claim5, wherein the location of interest is in the open zone in the wellborehole.
 8. The method of claim 1, wherein the step of locatingcomprises the step of locating array of surface electromagnetic fieldreceivers at spaced positions conforming to dimensions of a reservoir ofinterest.